In wireless communication apparatuses based on a Time Division Duplex (hereinafter, abbreviated as ‘TDD’) method, a communication time is divided by a constant time (transmitting period and receiving period), and transmitting and receiving are alternately carried out during the transmitting period and the receiving period respectively.
In the case of the TDD method, it is possible to carry out the transmitting and the receiving by use of the same carrier frequency. Accordingly, TDD has advantages that usage efficiency of frequency band is high, and it is possible to carry out asymmetric communication, and it is possible with ease to carry out the beam forming by use of information that is provided by a terminal using the same carrier frequency.
Here, the asymmetric communication means communication whose ratio of the transmitting period to the receiving period is not 1 to 1. As a specific example, there is a communication form in which a time length of uplink communication from a terminal to a base station and a time length of downlink communication from the base station to the terminal are different each other.
As a wireless communication service using the TDD method, PHS (Personal Handy-phone System), TD-CDMA (Time Division-Code Division Multiple Access), the mobile WiMAX (Worldwide Interoperability for Microwave Access), TD-LTE (Time Division-Long Term Evolution) or the like is exemplified.
In the case of the wireless communication apparatus based on the TDD method, a transmitting operation and a receiving operation are each carried out exclusively. As a result, it is possible to share an antenna and the like as a transmitter/receiver unit. As the wireless communication apparatus that shares the transmitter/receiver unit, for example, a RF transmitter/receiver device is described by PTL (Patent Literature) 1. To share the transmitter/receiver unit is carried out also by a wireless electric power transmitter apparatus (for example, refer to PTL 2) in addition to the wireless communication apparatus.
FIG. 12 shows a configuration of a typical wireless communication apparatus based on the TDD method.
A configuration and a work of a transmitter unit will be shown in the following.
A digital signal that is generated by a transmitting signal processing unit 9 is converted into an analog signal by a DA (Digital to Analog) conversion unit 91, and the analog signal is superimposed on to a RF (Radio Frequency) carrier by a modulation unit 4. The generated RF modulation signal is amplified up to a predetermined output level by a high-level output amplifier (Power Amplifier: hereinafter, abbreviated as ‘PA’) 10. Afterward, the amplified RF modulation signal is emitted by a shared antenna 3 through a transmit/receive switch unit 2.
A configuration and a work of a receiver unit will be shown in the following.
The RF modulation signal that is received by the shared antenna 3 passes through the transmit/receive switch unit 2 and is amplified by a low noise amplifier (hereinafter, abbreviated as ‘LNA’) 6.
Then, the RF modulation signal is converted into an intermediate frequency by a demodulation unit 7, and furthermore only a desired channel signal that exists in the RF modulation signal is selected. The analog signal that is generated by the selection is converted into a digital signal by an AD (Analog to Digital) conversion unit 81, and a received signal processing unit 8 carries out the digital signal processing to the digital signal to regenerate information.
In the case of the wireless communication apparatus based on the TDD method, it is possible to separately install a transmitting antennas and a receiving antenna, but in many cases, a shared antenna is used for transmitting and receiving.
In the case that the wireless communication apparatus carries out the transmitting, the transmit/receive switch unit 2 connects the shared antenna 3 and the transmitter unit and disconnects the shared antenna 3 and the receiver unit. In the case that the wireless communication apparatus carries out the receiving, the transmit/receive switch unit 2 connects the shared antenna 3 and the receiver unit and disconnects the shared antenna 3 and the transmitter unit.
An example of a specific configuration of the transmit/receive switch unit 2 is shown in FIG. 13. According to the example of the configuration, the transmit/receive switch unit 2 includes two PIN (Positive Intrinsic Negative) diodes 21 and 23, and a quarter wavelength transmission line (impedance transformer) 22. By setting a control signal Vcont to be High during the transmitting, both of the diodes 21 and 23 are set to be in a state of forward bias. At this time, a high frequency impedance of a path from an output of PA 10 to the antenna 3 becomes zero in an ideal case, and consequently a transmitted signal provided by PA 10 is emitted from the antenna 3 as a radio wave.
On the other hand, a high frequency impedance of a path from PA 10 to LNA 6 is infinite in an ideal case since one end of the quarter wavelength transmission line 22 is connected with the ground by the diode 21. Therefore, leakage of the transmitted signal from a transmitter side to a receiver side is restrained.
In contrast, during the receiving, by setting the control signal Vcont to be Low, both of the diodes 21 and 23 enter into a state of reverse bias. Therefore, a high frequency impedance seen from the antenna 3 to the receiver side becomes zero in an ideal case and a high frequency impedance seen from the transmitter side to the antenna 3 becomes infinite in an ideal case. That is, a receiving signal that is received by the antenna 3 is inputted into LNA 6, and an output terminal of PA 1 and the antenna 3 are disconnected each other.
By the way, in the case of a wireless communication apparatus that includes a plurality of the wireless communication methods or a plurality of carrier frequencies, it is necessary to switch a plurality of high frequency ports. According to the transmit/receive switch method in FIG. 13 that uses the diodes 21 and 23, and the quarter wavelength transmission line 22, a size is large and a configuration is complex, and consequently the transmit/receive switch method in FIG. 13 is unsuitable for multi-port configuration. Therefore, in the case of a wireless communication apparatus that includes a plurality of ports, a switch method that uses a semiconductor switch shown in FIG. 14 is widely used.
According to a configuration shown in FIG. 14, by setting a control signal Vcont 1 and a control signal Vcont 2 to be High and Low respectively during the transmitting, a field-effect transistor (hereinafter, abbreviated as ‘FET’) 25 is set to be in a conductive state, and FET 26 is set to be in a non-conductive state. Moreover, by setting the control signal Vcont 1 and the control signal Vcont 2 to be Low and High respectively during the receiving, FET 25 is set to be in the non-conductive state, and FET 26 is set to be in the conductive state. As a result, the antenna 3 and one of the transmitter side and the receiver side are connected, and the antenna 3 and the other are disconnected.
However, in the examples shown in FIG. 13 and FIG. 14, to make the diode 21 grounded is actually not perfect at a high frequency band. Moreover, a resistance at a time when FETs 25 and 26 are off is not infinite. Therefore, impedance of the transmit/receive switch unit 2 is not infinite, and consequently perfect isolation cannot be carried out. Therefore, during the transmitting, finite leakage of electric power from the transmitter unit to the receiver unit exists. The leakage from the transmitter unit to the receiver unit makes LNA saturated and makes a load changed to degrade receiving characteristics.
Moreover, also in the examples shown in FIG. 13 and FIG. 14, passage resistances of the diode 23 and FET 25 are actually not zero even when the diode 23 and FET 25 are in the conductive state. As a result, a finite electric power loss exists.
Since, in general, PA is one of blocks which consume electric power most among wireless apparatuses, an electric power loss of a PA output causes increase of electric power consumption of the whole apparatus even if the electric power loss of the PA output is minute.
A measure for solving the above-mentioned problems that are caused by the leakage from the transmitter unit to the receiver unit, and the electric power loss of PA is described, for example, in PTL 3. In the case of a digital cellular phone that is described by PTL 3, impedance that is formed by viewing a transmitter side from a transmit/receive connection point of the antenna is high during the receiving, and impedance that is formed by viewing a receiver side from the transmit/receive connection point is high during the transmitting.
Specifically, as shown in FIG. 2 of PTL 3, an electric power supply of a collector in a common emitter PA that includes a bipolar transistor is turned on, and an electric power supply of a collector in a common emitter LNA is turned off. Furthermore, a length L of a transmission line that connects an end of antenna and LNA is adjusted so that impedance that is formed by viewing the receiver side from the end of the antenna may be large. In contrast, the electric power supply of the collector in PA is turned off, and the power supply of the collector in LNA is turned on during the receiving. Furthermore, a length L′ of a transmission line that connects the end of the antenna and PA is adjusted so that impedance that is formed by viewing the transmitter side from the end of the antenna may be large.
In the case of the digital cellular phone that is described by PTL 3, since electric power of PA is turned off during the receiving by the above-mentioned configuration, there is no case that the transmitter side causes disturbance for the receiver side. Moreover, in the case of the digital cellular phone that is described by PTL 3, the transmit/receive switch unit does not use any elements such as the diode and FET, the electric power loss is small and efficiency of PA is improved.
A method for solving the above-mentioned problems is described also by PTL 4. In the case of a wireless communication apparatus that is shown in FIG. 1 of PTL 4, both of a drain voltage Vd and a gate voltage Vg of a common source PA including FET are simultaneously turned off during the transmitting. By virtue of the control, an operation of PA is stopped quickly and perfectly to prevent the receiver side from being interfered by the transmitter side and to improve efficiency of the electric power.
Recently, there are some cases that a switching amplifier (D class amplifier) is used as a modulation power supply of PA based on the envelope tracking method or the like (for example, refer to PTL 5).
The switching amplifier has structure that two switching elements are arranged in a form of series-connection and connected between an electric power supply and the ground terminal. Since it is always set that one out of two switching elements is in a cut-off state, and the other is in a conductive state, a current does not flow basically between the electric power supply of the switching amplifier and the ground terminal. Therefore, the switching amplifier has an advantage that electric power consumption is small.
Here, an output signal of the switching element is a pulse signal. Then, the switching amplifier of PTL 5 includes a low-pass filter that removes a high frequency component of a pulse signal so that an output of the switching amplifier may be used as the electric power supply of PA, and outputs the filtered pulse signal as an analog signal.